1. Field of the Invention
This invention relates to signal processing circuits for improving the signal-to-noise ratio of a recorded and reproduced information signal, and more particularly is directed to a signal processing circuit for suppressing or eliminating low- and high-frequency noises superimposed on an information signal in the recording and/or reproducing playback thereof.
2. Brief Description of the Prior Art
In order to use recording tape most efficiently, it is desirable to record information signals on the tape while the tape is transported at a low speed. However, the level of noise superimposed on the information signals during recording and reproducing is greatly increased at low transport speed.
To suppress or eliminate such noise, several systems have been proposed, such as the noise reduction systems disclosed in U.S. Pat. No. 3,665,345 and in U.S. Pat. No. 3,911,371.
U.S. Pat. No. 3,665,345 discloses a wide band noise reduction system in which the information signals are passed, without processing, through a main signal path between an input terminal and an output terminal, and in which an additional path for the information signals filters and limits signals passing between the input and output terminals. For low amplitude signals, the noise reduction system of U.S. Pat. No. 3,665,345 acts as an all-pass filter, while for high-amplitude signals, the system acts as a high- or as a low-pass filter. In one version of such noise reduction system, two additional paths are provided, with one of such paths blocking those signals lower than about 100 Hz and the other additional path passing only those signals lower than 2 KHz. The extreme opposite portions of the voice spectrum are respectively blocked in the two additional paths to reduce noise modulation effects. In such a noise reduction system, the frequency responses of the additional paths overlap for substantial portions of their frequency bands. Thus, there is a lack of independence of the high-frequency and low-frequency portions of the overall frequency response of such a noise reduction system.
U.S. Pat. No. 3,911,371 describes a technique to increase the signal-to-noise ratio of an input information signal by compressing the dynamic range of the signal. Such a technique involves using a variable filter in which a control signal, based upon the amplitude of the information signal as a whole, controls the frequency characteristic of the filter. The noise reduction technique according to U.S. Pat. No. 3,911,371 also lacks independence between the frequency response characteristic for high-frequency signals and that for low-frequency signals. While such technique disclosed in U.S. Pat. No. 3,911,371 has proved more or less satisfactory for information signals recorded on a tape which is transported at a relatively high speed, noise signals, especially in a low-frequency band, are not completely reduced if a lower tape transport speed is used.
The dynamic range compression technique makes use of the fact that noise is particularly objectionable when the amplitude of the information signal is low and does not mask the noise. According to such technique, low amplitude signals are amplified more than high amplitude signals before these signals are applied to a part of the transmission medium (i.e., recording tape) where a specific type of noise is likely to be added to the original signal.
The advantage of dynamic range compression in a recording and reproducing system is the suppression of low-amplitude noise signals, such as might be introduced by the recording medium itself or might be picked up by the recording and/or reproducing transducer. In the recording system, low-amplitude signals are given more gain than high-amplitude signals resulting in dynamic-range compressed signals, and then these compressed signals are recorded on a recording medium. Thereafter, a reproducing system amplifies the recorded signals so as to afford less gain to the relatively low-amplitude signals than to the relatively high-amplitude signals, thereby to restore the dynamic range of the original signal. In other words, the overall recording and reproducing system affects the signals uniformly by having the reproducing system compensate for modification in signal amplitude introduced by the recording system. Thus, low-amplitude noise signals introduced during recording or reproducing pass only through those parts of the total transmission path in which they are amplified relatively little. If the foregoing technique is applied to the higher-frequency portion of the information signal bandwidth, high-frequency noise, such as tape hiss, can be eliminated. Similarly, if this technique is applied to the lower-frequency portion of the frequency band of the information signal, low frequency noise, such as hum, can be eliminated.
When a lower tape speed is used, it is especially important to be able to eliminate both the low-frequency noise (hum) and the high frequency noise (hiss). However, in order to achieve a reduction in both the high and low frequency noises, it is necessary to control the dynamic range of the lower frequency signals and the dynamic range of the higher-frequency signals independently from one another.